It is well known in the prior art train vehicle control system operation to compare a transmitted digital speed code signal with a received digital speed code signal in relation to vehicle occupancy determination in a defined track circuit signal block to which that speed code signal is supplied, for controlling the speed of a train vehicle present within that signal block. It is also well known to supply frequency tone speed code signals to a track circuit signal block to control the movement speed of train vehicle within that signal block, with a particular frequency tone being supplied to the signal block for the open loop control of the desired speed of a train vehicle moving within that signal block.
The BART automatic train control system as described in an article published in the September 1972 Westinghouse Engineer at pages 145-151, transmitted a digital speed code signal to one end of a predetermined track circuit signal block and that same digital speed code signal was received at the opposite end of the signal block for the purpose of detecting signal block occupancy by a train vehicle. For this purpose the received speed code signal was compared with the transmitted original speed code signal.
In other prior art train vehicle control systems, where there is no multiplex signaling system and no digital bits of speed code signals, there is provided a unique frequency tone or carrier frequency for controlling the speed of the train vehicles. There is no digital signal that can be compared, and the frequency tone amplitude modulates a carrier frequency signal to be either ON or OFF on a full 100% modulated basis.
When transmitting speed code signals into the track of a steel wheel and steel rail system, it is well known to compare at some point the speed code signal that is sent with that received within a track circuit signal block or zone. This comparison serves to establish whether or not the signal block is occupied by a train vehicle and in addition, by comparing the speed code signal, safety is increased by virtually eliminating the likelihood of receiving the same speed code signal from an adjacent signal block when a particular signal transmitter fails or track bonds become broken, which condition can be dangerous since a valid train vehicle occupancy may not be detected as such. In addition, it is known to employ frequency separation to separate the speed code signals in relation to adjacent signal blocks, as described in U.S. Pat. No. 3,532,877 of G. M. Thorne-Booth, which discloses a serial six-bit speed code signal and the received speed code signal is compared bit by bit with the transmitted speed code signal.
In a train vehicle control situation where serial bit speed code signals are not utilized, the conventional method of frequency tone coded speed code signals is used.